1. Field of the Invention
This invention relates generally to communication devices and, more particularly, to processing of received signals in mobile communication devices.
2. Description of the Related Art
Mobile communication devices can include a wide range of devices, including cellular telephones, personal communication devices, and cordless telephones. Such devices include a handset that receives transmitted communication signals from a base station or cell station. The transmitted signal comprises a baseband voice signal that was modulated onto a carrier frequency to produce a signal that can be transmitted over an assigned band of the radio frequency spectrum. At the receiving handset, the modulated signal is received. The handset must then remove the carrier frequency to retrieve the baseband voice signal. A similar sequence of processing occurs in sending a voice signal from the handset to the base station.
More recently, the processing of the received signal occurs in the digital domain, although the transmitted signal is received as an analog signal. Thus, the received signal must undergo an analog-to-digital (A/D) conversion process. As the mobile communication device is moved about, the received signal strength can vary. For proper digital conversion, it is necessary to maintain control over the signal magnitude. Therefore, processing of the received signal usually includes amplification of the received signal, determination of received signal strength, and received signal gain control. In addition, the demodulation of the received signal for removal of the carrier frequency requires internal generation of a signal at the carrier frequency. This typically is achieved using a master clock signal that is generated in the handset. Inaccuracy in the master clock signal or parts imperfection can cause a DC offset in the generated signal and also result in an offset in the demodulated signal, which can prevent proper demodulation of the received signal.
Gain control of the received signal is typically achieved with a calculation of signal power by a standard computation involving I and Q terms of the A/D conversion process. In particular, the signal power is determined to be equal to the sum of the I.sup.2 and Q.sup.2 terms, where I and Q are A/D conversion terms of the received signal. Under conditions of fading and spectrum noise that are typical for a mobile communications device, such a signal power calculation can be difficult and unreliable. The response time to variations in the received signal strength can be unsatisfactory, and oscillation of gain adjustment can occur, causing further problems in signal demodulation. Response time can be faster and production costs can be lower if the processing is kept as simple as possible.
It should be apparent that signal gain control and associated processing performance can be improved and costs minimized with careful processing schemes for the received signal in a mobile communications device. It also would be advantageous if such improved performance would extend to better determination of received signal strength and superior generation of the master clock signal for better demodulation of the received signal and removal of the carrier frequency from the received signal.
From the discussion above, it should be apparent that there is a need for a mobile communications device with improved processing of the received signal without complex processing requirements. The present invention fulfills this need.